Thermophotovoltaic technology

ABSTRACT

A high output, narrow band thermally energized radiation source comprises a rare earth oxide radiator member that has a cross-sectional dimension in the range of five to thirty micrometers, the rare earth oxide radiator member, when heated to about 1700° C., having a concentrated radiated flux over the 400-2500 nanometer wavelength range such that at least 50% of the radiated flux is within a spectral band that is less than 400 nanometers wide.

This application is a continuation-in-part of my prior pendingapplication Ser. No. 529,016 filed Sept. 2, 1983, now abandoned.

This invention relates to radiation sources and more particularly tosources of the thermally excited type in which radiation is emitted froma heated element, and to thermophotovoltaic devices.

Radiation sources of the thermally excited type such as incandescentlamps in which light is emitted from a highly heated resistance wire andincandescent mantles of the Welsbach type have long been known. Suchradiation sources generally have characteristics of the "black body", ormore realistically "gray body", type and emit radiation over a broadspectral band. In accordance with one aspect of the invention, there isprovided a thermally excited radiation source that has a narrow peak inthe spectral profile of its radiated flux and skirt portions of theradiated flux profile on either side of the narrow peak are suppressedso that the emitted radiation has a concentrated spectral distribution.

Such a thermally excited narrow band radiation source may have a varietyof applications and, for example, may usefully be coupled to aphotovoltaic cell to provide a thermophotovoltaic device. Radiation thatis absorbed by a photovoltaic cell in the neighborhood of a potentialbarrier, usually a pn junction, gives rise to separated electron-holepairs which create an electric potential. The photocell conversionefficiency is a function of the band gap (in electron volts) and thetemperature of the particular photocell material. Among the known typesof photocell material are silicon, which has a band gap of about 1.1electron volts, equivalent to a wavelength of about 1150 nanometers; andgermanium, which has a band gap of about 0.7 electron volt, equivalentto a wavelength of about 1800 nanometers. In a thermophotovoltaicdevice, a close match between the spectrum of photon energy radiatedfrom the radiation source and the electron production threshold of thephotovoltaic cell results in a greater amount of energy which isabsorbed by the photovoltaic cell being converted to electrical energyand a minimal amount being converted to heat. Silicon photovoltaic cellshave relatively low conversion efficiency in direct sunlight, in partbecause the specific spectral energy of solar radiation does not providea good spectral match with the response of a silicon photovoltaic cellas that portion of solar radiation with wavelengths longer than 1100nanometers is useless to the silicon cell photovoltaic conversionprocess and generates heat in the cell requiring an increased effort forcooling to keep the cell at its best performance, and as the maximumspectral radiance in sunlight occurs at about 500 nanometers whichcorresponds to a photon energy of 2.5 electron volts, while only 1.1electron volts are required to produce the hole-electron pairs insilicon which contribute to external current flow and power output. Thesurplus energy of photons in the spectral region below 1100 nanometersis also converted to heat in the cell. While Welsbach mantles have beenproposed for use in thermophotovoltaic energy conversion systems, suchuses are not particularly efficient as such mantles generate substantialamounts of radiation throughout a spectral region that extends from thevisible well into the infrared. Other proposed thermophotovoltaic energyconversion systems have used reflector and rare earth active filterarrangements.

In accordance with one aspect of the invention, the rare earth oxideradiator member of the narrow band thermally energized radiation sourcehas a cross-sectional dimension in the range of five to thirtymicrometers, and that rare earth oxide radiator member, when heated toabout 1700° C., has a concentrated radiated flux over the 400-2500nanometer wavelength range such that at least 50% of the radiated fluxis within a spectral band that is less than 400 nanometers wide. Theradiation source may be thermally excited by various techniquesincluding, for example, electrical energy or combustion of liquid orgaseous fuels such as hydrogen, natural gas, propane, butane, isobutaneor gasoline.

In preferred embodiments, the narrow band thermally excited radiationsource is composed of interlocked fibers of at least one oxide of a hostrare earth metal selected from a class consisting of erbium, holmium,neodymium, and ytterbium, the radiated flux of the radiation sourcehaving a full width at half maximum (at 1/2 the maximum radiated flux ofthe source) of less than 400 nanometers. The relative spectralirradiance profiles of preferred radiators also have suppressed skirtcharacteristics such that at wavelengths 500 nanometers above and belowthe peak wavelength, the skirts have radiated fluxes that are less thanten and more preferably less than five percent of the profile peakradiated flux. In particular embodiments, the radiation source is aself-supporting rare earth oxide fiber mantle that defines a hollowspace, and that is secured on a support tube by an integral shrunkenskirt portion.

In accordance with another aspect of the invention there is provided athermophotovoltaic device that includes a photocell and a radiator ofrare earth metal oxide material disposed in optically coupled relationto the photocell. Such thermophotovoltaic devices may be used in powergeneration, topping cycles, cogeneration, or communication applications,for example. The radiator and photocell may be close coupled, forexample in the same housing, or spaced apart with the radiation from theradiator focused on the more remotely located photocell, or coupled asby means of fiber optic technology. The thermophotovoltaic device alsoincludes means for thermally exciting the radiator to cause it to emitradiation in a spectral irradiance profile that has a radiated flux peakwith a full width at half maximum of less than 400 nanometers, theradiated flux peak being less than 400 nanometers below (on the higherenergy side of) the electron production threshold of the photocell. Thephoton conversion efficiency of preferred thermophotovoltaic devices ismore than fifty percent.

In accordance with still another aspect of the invention a radiator ofrare earth metal oxide material is thermally excited at a temperature inthe range of 1500°-2000° C. to cause the radiator to emit radiation in aspectral irradiance profile that has a narrow radiated flux peak thathas a full width at half maximum of less than 400 nanometers andpreferably less than 200 nanometers, and suppressed skirtcharacteristics such that at wavelengths in the range of 300-500nanometers above and below the flux peak, the radiated flux levels ofthe skirts are less than ten and preferably less than five percent ofthe peak radiated flux.

In particular thermophotovoltaic device embodiments, the thermalexcitation system includes a liquid hydrocarbon fuel supply, a conduitconnected to the fuel supply that has an outlet port aligned with therare earth metal oxide radiator, a fuel control for controlling the flowof fuel through the conduit to the radiator, and an igniter mechanismfor igniting the fuel. Particular radiators are self-supporting rareearth metal oxide fiber mantles that are composed of metal oxidemulti-filament strands with cross-sectional strand dimensions in therange of 0.05-0.3 millimeter and filament cross-sectional dimensions inthe range of five to thirty micrometers, such mantles having an overalllength in the range of about 0.5-4.0 centimeters and a diameter in therange of about 0.5-2.0 centimeters. A reflector system mayadvantageously be employed for collecting, directing and concentratingthe radiated flux from the mantle to the photocell, and radiationtransmitting thermal isolation structure may be positioned between themantle (or mantles) of the radiation source and the photovoltaic cellarray. Particularly useful thermophotovoltaic devices include anytterbia mantle coupled to a silicon type photocell and an erbia mantlecoupled to a germanium type photocell.

Mantle arrangements such as multiple mantles or mantles of more complexgeometry such as pleated structures that are designed to radiate moreenergy without a corresponding increase in convection loss may also beemployed in thermophotovoltaic devices in accordance with the invention.Additional efficiency enhancement may be obtained by a regeneratorthrough which the hot convection gas is routed to warm the incomingcombustion air.

Other features and advantages of the invention will be seen as thefollowing description of particular embodiments of the inventionprogresses, in conjunction with the drawings, in which:

FIG. 1 is an enlarged view of a mantle type radiation source and itssupport in accordance with aspects of the invention;

FIG. 2 is a magnified view of a portion of the rare earth oxide fabricof the mantle shown in FIG. 1;

FIG. 3 is a graph indicating spectral characteristics of an ytterbiamantle type radiation source in accordance with the invention, and of apredominately thoria mantle type radiation source of similarconfiguration;

FIG. 4 is a diagram of a thermophotovoltaic device in accordance withaspects of the invention;

FIG. 5 is a diagrammatic front view of the photocell array employed inthe thermophotovoltaic device shown in FIG. 4;

FIG. 6 is a graph indicating spectral characteristics of anotherytterbia radiation source in accordance with aspects of the inventionand spectral characteristics of a commercially available predominatelythoria mantle;

FIG. 7 is a graph indicating spectral characteristics of anotherytterbia radiation source in accordance with the invention energizedwith a hydrogen flame;

FIG. 8 is a graph indicating spectral characteristics of anytterbia-ceria radiation source; and

FIG. 9 is a graph indicating spectral characteristics of an erbiaradiation source in accordance with aspects of the invention.

DESCRIPTION OF PARTICULAR EMBODIMENTS

The diagrammatic view of FIG. 1 shows a radiation source 10 of themantle type that is mounted on support tube 12 of mullite or othersuitable heat resistant material. Support tube 12 has an outer diameterof about five millimeters and an inner diameter of about threemillimeters. Mantle 10 is a self-supporting ytterbia fiber fabricstructure that defines a hollow chamber 14 of about seventy cubicmillimeters volume with tip 16 about 1/2 centimeter above the upper endsurface 18 of support tube 12. The lower portion 20 of the mantle fabricis about 1/2 centimeter in length and is firmly shrink-secured to theouter surface of support tube 12. The mantle fabric is formed ofytterbia multi-filament strands 22 in an open knit array with openings24 such that the open area of the fabric is about 60%. Thecross-sectional dimensions of the individual ytterbia filaments 26 (FIG.2) of strands 22 are in the range of about five-ten micrometers, and thestrands 22 have cross-sectional dimensions in the order of about 0.1millimeter with openings 24 having dimensions of about 0.5 millimeter.

A process for manufacturing mantle 10 is as follows. Continuous lowtwist, low tenacity (highly reticulated), viscose rayon yarn (150denier/60 filament) is knitted into a continuous tubular sleeve using aLamb circular string knitter (Model ST3A/ZA) with a 2.2 centimeterdiameter arbor and 24 needle capacity using 12 equally spaced needles inthe arbor. The yarn is knitted into a continuous tubular sleeve withtension on both the yarn and the knitted sleeve to attain about threestitches per linear centimeter of tensioned sleeve, and the continuouslength of knitted sleeve is wound onto a take-up spool.

An imbibing solution is formed by dissolving hydrated ytterbium nitrate(Yb(NO₃)₃.4 H₂ O) (reagent grade) in distilled water (together with asmall amount of a non-ionic wetting agent such as Triton X-100) toprovide a solution 1.75 molar in ytterbium nitrate.

A knitted rayon sleeve unit, about thirty centimeters long, is immersedfor about ten minutes in the imbibing solution at room temperature, withgentle agitation to promote penetration of the imbibing solution intothe rayon fibers. After the ten minute imbibition, the sleeve is removedfrom the solution, squeeze dried and then transferred to a plasticcentrifuge tube. The sleeve is then centrifuged for ten minutes at about200 g's to remove surface liquid. It is convenient to secure a metalscreen halfway from the bottom of the centrifuge tube so that liquiddoes not rewet the surface of the sleeve during or after centrifugation.

After centrifugation, the imbibed sleeve is formed into mantle sockswith aid of a Teflon sock-shaping rod that is fourteen millimeters indiameter and has a hemispherical end. A seven centimeter length ofimbibed sleeve is slipped over the shaping rod, and tied off at thehemispherical end of the shaping rod with a piece of treated yarnunraveled from the bottom of the knit sleeve. The shaped sock is thendried with a flow of hot (about 90° C.) air, slipped off the shapingrod, cut to about 3.6 centimeters length, and then hung on a fixturethat includes a series of upstanding mullite posts (spaced at aboutthree centimeter intervals) on a mullite base. Each post has a diameterof about three millimeters and a length of about 3.7 centimeter andreceives a support tube 12 and spacer, the top of tube 12 being spacedabout five millimeters below the top of the post so that the top of theshaped sock is supported on the mullite post and the lower portion ofthe shaped sock extends over the support tube 12. Optionally a ring ofsodium silicate that has been pretreated by heating tube 12 to about900° C. may be carried by tube 12.

The fixture with knitted imbibed socks hung over the support sleeves 12on the fixture posts is then subjected to a firing procedure to convertthe ytterbium nitrate imbibed cellulosic mantle socks into densifiedytterbia mantles.

In the firing sequence, the fixture with imbibed socks is placed in atubular oven that is about 1.2 meters in length and about fivecentimeters in inner diameter. At ambient temperature (about 25° C.),the oven is flushed with tank nitrogen at a flow rate of 200 cubiccentimeters per minute (a flow velocity of about ten centimeters perminute), and with this inert atmosphere in the oven, the oventemperature is increased at a rate of 400° C. per hour. The mantlefabric undergoes denitration at about 160° C. At this point the fabriccolor changes rapidly from white to golden tan. Immediately after thiscolor change, oxygen is added to the nitrogen flow at a rate of aboutfive cubic centimeters per minute. Heating continues at the same rate toa temperature of about 320° C. During this time the color continuouslychanges from golden tan to dark brown or black with modest shrinkage(about 10%) of the fabric, which indicates partial decomposition of theorganic material. The oven temperature is then held at about 320° C. forabout one and one-half hours and the mantle color turns from black tolight gray or white. During this soaking interval, the remaining carbonis oxidized and driven off and the mantle shrinks to about 1/3 itsoriginal dimensions with its lower portion 20 shrunk onto tube 12essentially as shown in FIG. 1. At the end of the soaking interval, theflow of oxygen is increased to fifty cubic centimeters per minute (a gasmixture of 20% oxygen) and the oven temperature is increased over atwenty-four minute interval to a temperature of 900° C. The heater isthen turned off and the oven is rapidly cooled to ambient temperature.After cooling, each mantle subassembly is removed from its storageholder post and is exposed to a burning mixture of isobutane and air (atan estimated mantle temperature of about 1700° C.) for five minutes tofurther shrink and densify the ytterbia fabric.

The resulting ytterbia fabric, in visual appearance, substantiallyretains characteristic physical textile attributes of its precursorrayon fabric, although it is substantially reduced in dimension. Thisytterbia fabric has relatively high density, is flexible, and has aminimal defect (flaws) microstructure, a concentrated radiated fluxprofile and, in the described mantle configuration, withstands impactloads of two thousand g's.

The graph of FIG. 3 shows a relative spectral irradiance profile 30 ofmantle 10, over a 400-2500 nanometer spectral range when that mantle isexposed to an air-isobutane flame (7 sccm isobutane, 113.6 sccm air asmeasured with Tylan controllers--an oxidant fuel ratio of 16.2). Profile30 was measured over an interval of 3.2 hours with a Beckman DK-2A prismspectroradiometer operated in single-beam mode with an adjustable slitwidth. Theoretical photovoltaic response of silicon is indicated bycurve 32 in FIG. 3; and the relative spectral irradiance profile of apredominantly thoria mantle, of similar physical shape to mantle 10 thatwas thermally energized with an an air-isobutane flame (7 sccmisobutane, 191.7 sccm air--an oxidant-fuel ratio of 27.4) and measuredin the same manner and over the same wavelength range as profile 30, isindicated by curve 34.

The radiated flux profile 30 of ytterbia mantle 10 has a radiation peak36 at about 985 nanometers (the spectroradiometer being set to have aslit width of about 0.6 mm at that wavelength) with half intensitypoints 38, 40 at about 890 nanometers and 1050 nanometers respectivelyso that peak 36 has a full width at half maximum of about 160nanometers. The lower skirt 42 of profile 30 of mantle 10 has measuredradiated flux levels of less than one-half percent of the peak from 400nanometers to 725 nanometers, and the upper skirt 44 of profile 30 hasradiated flux levels of one percent or less at spectral wavelengths from1300 nanometers to 2500 nanometers.

In contrast, the Welsbach type thoria mantle of similar physical sizeand construction has a broad band spectral profile as indicated by curve34, that profile indicating a radiated flux at 650 nanometers (in thevisible region) that is about ten percent of the peak radiated flux;radiated fluxes of about 35% of peak at 800 nanometers, 51% of peak at900 nanometers, 62% of peak at 1000 nanometers, 81% of peak at 1100nanometers in the near infrared region; a peak radiated flux at about1400 nanometers in the far infrared region (the spectroradiometer beingset to have a slit width of about 0.8 mm at that wavelength); andradiated flux intensities of about 87% at 1800 nanometers, 64% at 2100nanometers and 50% at 2400 nanometers. It will be seen that the ytterbiamantle 10 has a narrow well-defined radiation peak 36 in contrast withthe broad and diffuse radiation profile 34 of the Welsbach type mantle,as well as almost total suppression of skirts 42, 44 on either side ofpeak 36. The amplitude of peak 36, in absolute terms, is substantiallygreater than (about twice) the peak value of the broad band radiator(curve 34) (the area under curve 30 being about one-third the area undercurve 34 over the 500-2500 nanometer range). The ytterbia mantle 10 thusprovides a high output at about one micron, the wavelength of interest.

The thermophotovoltaic system shown in FIG. 4 incorporates ytterbiamantle 10. Support tube 12 is connected via a fuel delivery system tofuel reservoir 50 that contains isobutane fuel with outlet tube 52extending to pressure regulator 54. The high velocity jet atflow-through outlet orifice 56 (0.05 millimeter diameter) is directedthrough venturi 58 (a throat diameter of about one millimeter) toaspirate and mix air with the isobutane fuel for flow through supporttube 12 to mantle 10. Piezoelectric ignitor 60 or other suitableignition means is utilized to generate a spark after the air/fuelmixture delivery system is turned on to ignite the air isobutane fuelmixture at the mantle and provide a maximum flame temperature of about1900° C.

Mantle 10 is disposed in a polished and passivated aluminum reflector 62which collects and collimates the radiation emitted by mantle 10. Atubular reflector array 64 directs the radiation from mantle 10 onsilicon photovoltaic cell array 70 that is mounted on heat sinkstructure 72. Reflector array 64 is composed of a series of spaced,overlapping wall sections 66 with ventilation ports 68. Radiationtransmitting thermal isolation window 74 of glass or other suitablematerial is supported on one of the sections 66 and disposed betweenmantle 10 and photocell array 70 for isolating the hot combustion gas inthe mantle compartment from the photovoltaic cell array 70. The surfacesof window 74 preferably carry an anti-reflecting coating to lowerreflection losses. Preferably, the front surfaces of the photocells 76(FIG. 5) also carry an anti-reflection coating for the same reason thatwindow 74 is coated. To maximize photon conversion, silicon photocells76 preferably are about one millimeter in thickness. Provision of areflecting layer (for example of silver, aluminum or copper) on the backcontacts of the photocells 76 further enhances probability of photonconversion. Control system 78 may be connected between regulator 54 andphotocell array 70 for monitoring the output voltage and modulating thefuel flow by control of regulator 54 to control the electrical output ofthe photocell array 70.

The optical temperature of ytterbia mantles 10, measured with a Leeds &Northrup optical pyrometer (Catalog #8632-C) is in the range of1160°-1190° C. As the emissivity of the highly reflective mantlematerial (at the 660 nm optical pyrometer measuring wavelength) is lessthan 0.1, the actual temperature of mantle 10 is about 1700° C.

The electron production threshold of the silicon photovoltaic cells 76is 1150 nanometers, as indicated by curve 32 in FIG. 3. The ytterbiamantle 10, when heated as described above to about 1700° C., has itspeak 36 in close proximity to (about 165 nanometers from) siliconelectron production threshold 32 such that the electrical energyconversion efficiency of the system is high--80% of the total radiatedflux output of mantle 10 over the 400-2500 nanometer wavelength rangebeing convertible into electrical energy on the basis of the theoreticalsilicon cell response indicated in FIG. 3.

The graph of FIG. 6 shows the relative spectral irradiance profile 80,over the 400-2500 nanometer spectral range, of a second ytterbia mantlein accordance with the invention. That mantle is formed of a similartube of knit rayon that was imbibed in a 1.2 molar ytterbium nitratesolution and processed by uniformly torching the dried nitrateimpregnated rayon mantle with a blue propane flame to pyrolyze the rayonand shrink the mantle skirt onto its support tube. That ytterbiamantle-support tube assembly was energized with an air-isobutane flame(air/fuel ratio of 27:1) and its radiated flux was measured over the400-2500 nanometer spectral range with the same equipment as describedin connection with the mantle shown in FIGS. 1 and 2. This ytterbiamantle had a radiation peak 82 at about 970 nanometers (thespectroradiometer being set to have a slit width of about 0.6 mm at thatwavelength) with half peak intensity points 84 and 86 at about 875 and1060 nanometers respectively so that peak 82 has a width at half maximumintensity of about 185 nanometers. Measured radiated flux levels oflower skirt 88 were less than one-half percent from 450 to 625nanometers; and measured radiated flux levels of upper skirt 90 were twopercent or less at wavelengths from 1300 to 2500 nanometers. Peak 82 isoffset about 180 nanometers below silicon threshold 32. The siliconconversion efficiency of this ytterbia-isobutane mantle system was 74%.

Curve 92 is the relative spectral irradiance profile of a commercialColeman (Welsbach type) mantle energized in an air/propane flame. Itwill be seen that curve 92 has a broad spectral profile similar to curve34, curve 92 having a radiated flux level at 650 nanometers (the upperend of the visible range) of about 46% of its peak radiated flux;radiated flux levels in the near infrared region of about 62% of peak at800 nanometers, 71% of peak at 900 nanometers, and 85% of peak at 1000nanometers; a peak radiated flux at about 1400 nanometers (thespectroradiometer being set to have a slit width of about 0.3 mm at thatwavelength); and radiated flux outputs of about 80% of peak at 1800nanometers, 52% of peak at 2100 nanometers and 42% of peak at 2400nanometers. The silicon conversion efficiency of this Welsbachmantle-propane system is 23%.

Another mantle in accordance with the invention is a self-supportingytterbia fiber fabric structure that defines a hollow chamber of abouttwelve millimeters in diameter with its tip about 2.5 centimeters abovethe upper end surface of a brass fuel supply support tube to which it issecured, the support tube having an outer diameter of about twelvemillimeters and an inner diameter of about seven millimeters, the lowerportion of the mantle fabric sleeve being firmly shrink-secured to theouter surface of the support tube. The mantle fabric is composed ofytterbia multi-filament strands in a knit array that has an open area ofabout 30%, the cross-sectional dimensions of the individual ytterbiafibers of the strands of this mantle being about fifteen micrometers,and the strands having cross-sectional dimensions of about 0.2millimeter with openings having dimensions of about 0.3 millimeter.

This mantle was manufactured from continuous low twist, low tenacity(highly reticulated), viscose rayon yarn (300 denier/50 filament) thatwas knitted into a continuous tubular sleeve using a Lamb circularstring knitter (Model ST3A/ZA) with a 3.8 centimeter diameter arbor andsixty needle capacity using sixty equally spaced needles in the arborwith tension on both the yarn and the knitted sleeve to attain eightstitches per linear centimeter of tensioned sleeve. An imbibing solution1.57 molar in ytterbium nitrate was formed and a length of the knittedrayon sleeve was immersed for about one hour in the imbibing solution atroom temperature, with gentle agitation to promote penetration of theimbibing solution into the rayon fibers. After imbibition andcentrifugation, the imbibed sleeve was formed into mantle socks,disposed on brass support tubes, and processed by uniformly torching thedried nitrate impregnated rayon mantle with a blue propane flame topyrolyze the rayon and shrink the mantle skirt onto its brass supporttube.

That ytterbia mantle-support tube assembly was then exposed to anair-isobutane flame (91 sccm isobutane, 2460 sccm air as measured withTylan controllers--an oxidant fuel ratio of about 27:1), and itsresulting spectral irradiance profile was measured with a Beckman DK-2Aprism spectroradiometer operated in single-beam mode with thespectroradiometer set to have a slit width of about 0.035 mm over the400-2400 nanometer wavelength range. The resulting radiated flux profileof that ytterbia mantle had a radiation peak at about 1010 nanometers(offset about 140 nanometers below the silicon threshold) with halfintensity points at about 905 nanometers and 1080 nanometersrespectively so that its peak had a full width at half maximum of about175 nanometers. The lower skirt of the profile of the mantle hadmeasured radiated flux levels of less than one-half percent of the peak,and the upper skirt of the profile had radiated flux levels of sevenpercent or less at spectral wavelengths from 1300 nanometers to 2400nanometers. The silicon conversion efficiency of this ytterbia-isobutanemantle system was about 72%. The relative spectral irradiance profile100 of still another ytterbia mantle is shown in FIG. 7. The ytterbiafiber configuration and composition (a 1.74 molar ytterbia imbibingsolution was used) were similar to mantle 10. After the ytterbia mantlewas carefully burned off in propane, profile 100 was obtained byexposing the mantle to an air-hydrogen flame (maximum flame temperatureabout 2045° C.) with the burner stem orifice reduced to an innerdiameter of 0.25 millimeter to avoid flashback at the fuel flow rateemployed (a 1.52 millimeter burner orifice was used with isobutanefuels). The ytterbia mantle was exposed to a air-hydrogen flame (70 sccmair flow rate, 112 sccm hydrogen flow rate) and measured with theBeckman spectroradiometer operated in single beam mode with a 0.8millimeter slit width at the 965 nanometer peak. The resultingnormalized spectral irradiance profile 100 as shown in FIG. 7 had a peak102 at about 965 nanometers (offset about 185 nanometers from siliconthreshold 32), half intensity points 104, 106 at about 890 and 1050nanometers so that peak 102 had a full width at half maximum of about160 nanometers; a lower skirt 108 that had less than one-half percentmeasured radiated flux levels from 450 to 675 nanometers and an upperskirt 110 that had radiated flux levels of about one percent from 1500to 2300 nanometers (with the exception of a two percent radiated fluxlevel at 1900 nanometers). The silicon conversion efficiency of thisytterbia-hydrogen mantle system was 76%. The relative spectralirradiance profile of still another mantle system is shown in FIG. 8, aknitted rayon tube of the same configuration as that used for mantle 10being imbibed with a solution of 1.69 molar ytterbia nitrate and 0.035molar cerium nitrate and then pyrolyzed by burning off in propane. Itsradiated flux was then measured with the Beckman spectroradiometer withthe same techniques as employed with the preceding examples employing aslit width of 0.55 millimeter at the 960 nanometer peak. Profile 120 ofthat ytterbia-ceria mantle had a radiation peak 122 at about 960nanometers (the spectroradiometer being set to have a slit width ofabout 0.6 mm at that wavelength), half intensity points 124, 126 atabout 900 and 1050 nanometers respectively so that peak 122 has a fullwidth at half maximum of about 150 nanometers; a lower skirt 128 of lessthan one-half percent measured radiated flux levels from 450 to 525nanometers; about one percent of peak radiated flux at 650 nanometers;and an upper skirt 130 with radiated flux levels of six to eight percentfrom 1300 nanometers to 1900 nanometers and radiated flux levels ofthree percent or less from 2200 nanometers to 2500 nanometers. Mantleprofile 120 has a silicon conversion efficiency of 58%.

Shown in FIG. 9 is the normalized relative spectral irradiance profile140, over a 450-2500 nanometer spectral range, of an erbia mantle formedby imbibing a rayon sleeve similar to the sleeve used to form mantle 10with a 0.92 molar solution of erbium nitrate and then carefully torchingthe imbibed sleeve in propane to pyrolyze the rayon and shrink themantle skirt on its support tube. Profile 140 was obtained by exposingthat erbia mantle to a air-isobutane flame (air/fuel ratio of 26.2). Theresulting erbia mantle profile 140 as shown in FIG. 9 had a radiationpeak 142 at about 1500 nanometers (the spectroradiometer being set tohave a slit width of about 0.4 mm at that wavelength)--peak 142 beingoffset about 300 nanometers from germanium electron production threshold144; half intensity points 146 and 148 at about 1430 and 1570 nanometersrespectively so that peak 142 had a full width at half maximum of about140 nanometers; a lower skirt 150 that had less than one percent of peakradiated flux for the range of 450-750 nanometers, and a component 152at about three percent of peak intensity from 900 nanometers to 1000nanometers; and an upper skirt 154 that had radiated flux levels of twopercent or less from 1800 nanometers to 2500 nanometers. In a moreprecise measurement of profile 140 over the 1200-2000 nanometerwavelength range (the spectroradiometer being set to have a slit widthof about 0.04 mm)--peak 142 was located at about 1515 nanometers (offsetabout 285 nanometers from germanium electron production threshold 144;and half intensity points 146 and 148 were at about 1480 and 1545nanometers respectively so that peak 142 had a full width at halfmaximum of about 65 nanometers. The optical temperature of erbiamantles, measured with a Leeds & Northrup optical pyrometer (Model#8632-C) is in the range of 1400°-1450° C. As the emissivity of theerbia mantle material (at the 660 nm optical pyrometer measuringwavelength) is about 0.5, the actual temperature of the erbia mantle isabout 1600° C. This narrow band erbia radiation source couples well to agermanium photocell and such a thermophotovoltaic system has a germaniumphotocell conversion efficiency of about 77%. The erbia mantle providesa high output at about 1.5 microns, the wavelength of interest, incontrast to the lower absolute values of output peaks of comparablebroad band radiators of the type indicated by curve 34, for example.

While particular embodiments of the invention have been shown anddescribed, various modifications will be apparent to those skilled inthe art, and therefore it is not intended that the invention be limitedto the disclosed embodiments or to details thereof, and departures maybe made therefrom within the spirit and scope of the invention.

What is claimed is:
 1. A thermophotovoltaic device comprising aphotocell, a rare earth metal oxide radiator disposed in opticallycoupled relation to said photocell, and means for thermally excitingsaid radiator to cause it to emit radiation, said radiator, when heatedto 1700° C., emitting radiation in a spectral irradiance profile thathas a narrow radiated flux peak that is located less than four hundrednanometers below the electron production threshold of said photocell,said flux peak having a full width at half maximum of less than fourhundred nanometers and said relative spectral irradiance profile of saidradiator also having suppressed skirt characteristics such that, atwavelengths five hundred nanometers above and below said flux peak, theradiated flux levels of said skirts are less than ten percent of theradiated flux at said peak.
 2. The thermophotovoltaic device of claim 1wherein said device has a photon conversion efficiency of more thanfifty percent.
 3. The thermophotovoltaic device of claim 2 wherein saidradiator is a mantle composed of metal oxide filaments, each saidfilament having a cross-sectional dimension in the range of five-thirtymicrometers.
 4. The thermophotovoltaic device of claim 3 wherein saidmantle is composed of multi-filament metal oxide strands that have across-sectional dimension in the range of 0.05-0.3 millimeter.
 5. Thethermophotovoltaic device of claim 3 wherein said mantle has an overalllength in the range of about 0.5-4.0 centimeters and a diameter in therange of about 0.5-2.0 centimeters.
 6. The thermophotovoltaic device ofclaim 3 wherein said mantle is shrink-supported on a fuel supplyconduit.
 7. The thermophotovoltaic device of claim 2 wherein saidradiator is composed of filaments of an oxide of ytterbium, each saidfilament having a cross-sectional dimension in the order of tenmicrometers; and said photocell is of the silicon type.
 8. Thethermophotovoltaic device of claim 2 wherein said radiator is composedof filaments of an oxide of erbium, each said filament having across-sectional dimension in the order of ten micrometers; and saidphotocell is of the germanium type.
 9. The thermophotovoltaic device ofclaim 2 wherein said radiator is a mantle composed of metal oxidefilaments, each said filament having a cross-sectional dimension in therange of five-thirty micrometers, said photocell is of either thesilicon or the germanium type, and said radiator is composed offilaments of an oxide of ytterbium when said photocell is of the silicontype and of filaments of an oxide of erbium when said photocell is ofthe germanium type.
 10. The thermophotovoltaic device of claim 1 andfurther including a reflector system for collecting and directing theradiated flux from said radiator to said photocell.
 11. Thethermophotovoltaic device of claim 1 and further including radiationtransmitting thermal isolation means interposed between said radiatorand said photocell.
 12. The thermophotovoltaic device of claim 1 whereinsaid radiator, when heated to 1700° C., emits radiation in aconcentrated portion of the 400-2500 nanometer wavelength range suchthat at least 50% of the radiated flux is within a band less than 400nanometers in width, said flux peak has a full width at half maximum ofless than two hundred nanometers and said suppressed skirtcharacteristics are such that, at wavelengths in the range of three-fivehundred nanometers above and below said flux peak, the radiated fluxlevels of the skirts are less than five percent of the radiated flux atsaid peak.
 13. A thermophotovoltaic device comprising:a photovoltaicdevice of the germanium-type, said photovoltaic device having anelectron production threshold, a fuel supply, a fuel supply conduitconnected to said fuel supply and having an outlet port, a mantle oferbium oxide material supported on said fuel supply conduit adjacentsaid outlet port, said photovoltaic device being in optically coupledrelation to said mantle, a fuel control for controlling the flow of fuelto said mantle through said conduit, and an igniter mechanism forigniting said fuel to cause said mantle to emit radiation that has awavelength peak below said electron production threshold, said peakhaving a full width at half maximum of less than about 150 nanometersand said mantle, when heated to about 1700° C., having a concentratedradiated flux output over the 400-2500 nanometer wavelength range suchthat at least 50% of said radiated flux is within a band less than 400nanometers in width.
 14. The thermophotovoltaic device of claim 13wherein said fuel is a liquid or gaseous fuel such as hydrogen, naturalgas, propane, butane, isobutane, or gasoline.
 15. The thermophotovoltaicdevice of claim 13 wherein said mantle is composed of interlocked erbiumoxide fibers, each said fiber having a cross-sectional dimension in therange of five-thirty micrometers.
 16. The thermophotovoltaic device ofclaim 13 and further including a reflector system for collecting anddirecting the radiated flux from said mantle to said photovoltaicdevice.
 17. The thermophotovoltaic device of claim 16 and furtherincluding radiation transmitting thermal isolation means interposedbetween said mantle and said photovoltaic device.
 18. Athermophotovoltaic device comprising:a photovoltaic device of thesilicon-type, said photovoltaic device having an electron productionthreshold, a fuel supply, a fuel supply conduit connected to said fuelsupply and having an outlet port, a mantle of ytterbium oxide materialsupported on said fuel supply conduit adjacent said outlet port, saidphotovoltaic device being in optically coupled relation to said mantle,a fuel control for controlling the flow of fuel to said mantle throughsaid conduit, and an igniter mechanism for igniting said fuel to causesaid mantle to emit radiation, said mantle, when heated to 1700° C.,emitting radiation in a spectral irradiance profile that has a narrowradiated flux peak that is located less than four hundred nanometersbelow the electron production threshold of said photovoltaic device,said flux peak having a full width at half maximum of less than fourhundred nanometers and said relative spectral irradiance profile of saidmantle also having suppressed skirt characteristics such that, atwavelengths five hundred nanometers above and below said flux peak, theradiated flux levels of said skirts are less than ten percent of theradiated flux at said peak.
 19. The thermophotovoltaic device of claim18 wherein said fuel is a liquid or gaseous fuel such as hydrogen,natural gas, propane, butane, isobutane, or gasoline.
 20. Thethermophotovoltaic device of claim 18 wherein said mantle is composed ofinterlocked fibers of ytterbium oxide material, each said fiber having across-sectional dimension in the range of five--thirty micrometers. 21.The thermophotovoltaic device of claim 18 wherein said mantle iscomposed of multifilament ytterbium oxide strands that have across-sectional thickness dimension in the range of 0.05-0.3millimeters.
 22. The thermophotovoltaic device of claim 18 and furtherincluding a reflector system for collecting and directing the radiatedflux from said mantle to said photovoltaic device.
 23. Thethermophotovoltaic device of claim 22 and further including radiationtransmitting thermal isolation means interposed between said mantle andsaid photovoltaic device.
 24. A thermophotovoltaic device comprising:aphotovoltaic device, said photovoltaic device having an electronproduction threshold, a fuel supply, a fuel supply conduit connected tosaid fuel supply and having an outlet port, a metal oxide mantlesupported on said fuel supply conduit adjacent said outlet port, saidphotovoltaic device being in optically coupled relation to said mantle,a fuel control for controlling the flow of fuel to said mantle throughsaid conduit, and an igniter mechanism for igniting said fuel to causesaid mantle to emit radiation, said mantle, when heated to 1700° C.,emitting radiation in a spectral irradiance profile that has a narrowradiated flux peak that is located less than four hundred nanometersbelow the electron production threshold of said photovoltaic device,said flux peak having a full width at half maximum of less than fourhundred nanometers and said relative spectral irradiance profile of saidmantle also having suppressed skirt characteristics such that, atwavelengths five hundred nanometers above and below said flux peak, theradiated flux levels of said skirts are less than ten percent of theradiated flux at said peak.
 25. The thermophotovoltaic device of claim24 wherein said mantle, when heated to 1700° C., emits radiation in aconcentrated portion of the 400-2500 nanometer wavelength range suchthat at least 50% of the radiated flux is within a band less than 400nanometers in width, said flux peak has a full width at half maximum ofless than two hundred nanometers and said suppressed skirtcharacteristics are such that, at wavelengths in the range ofthree--five hundred nanometers above and below said flux peak, theradiated flux levels of the skirts are less than five percent of theradiated flux at said peak.
 26. A method of operating athermophotovoltaic device comprising the steps of disposing a radiatorof erbium oxide material in optically coupled relation to agermanium-type photocell, and thermally exciting said radiator to heatsaid radiator to a temperature in the range of 1500°-2000° C. to causesaid radiator to emit radiation in a spectral irradiance profile thathas a narrow radiated flux peak that is located less than four hundrednanometers below the electron production threshold of said photocell,said flux peak having a full width at half maximum of less than aboutone hundred fifty nanometers and said spectral irradiance profile ofsaid radiator also having suppressed skirt characteristics such that, atwavelengths five hundred nanometers above and below the flux peak, theradiated flux levels of the skirts are less than ten percent of theradiated flux at said peak, and said radiator-photocell system has aphoton conversion efficiency of more than fifty percent.
 27. The methodof claim 26 wherein said radiator is a mantle composed of erbium oxidestrands.
 28. The method of claim 27 and further including the steps ofsupporting said mantle adjacent the outlet port of a fuel supplyconduit, flowing fuel through said conduit to said mantle, and ignitingsaid flowing fuel to cause said mantle to emit radiation in saidspectral irradiance profile.
 29. The method of claim 26 and furtherincluding the step of providing a reflector system for directing theradiated flux from said radiator to said photocell.
 30. The method ofclaim 29 and further including the step of interposing radiationtransmitting thermal isolation means between said radiator and saidphotocell.
 31. A method of operating a thermophotovoltaic devicecomprising the steps of disposing a radiator of ytterbium oxide materialin optically coupled relation to a silicon type photocell, and thermallyexciting said radiator to heat said radiator to a temperature in therange of 1500°-2000° C. to cause said radiator to emit radiation in aspectral irradiance profile that has a narrow radiated flux peak that islocated less than four hundred nanometers below the electron productionthreshold of said photocell, said flux peak having a full width at halfmaximum of less than about four hundred nanometers and said spectralirradiance profile of said radiator also having suppressed skirtcharacteristics such that, at wavelengths five hundred nanometers aboveand below the flux peaks, the radiated flux levels of the skirts areless than ten percent of the radiated flux at said peak, and saidradiator-photocell system has a photon conversion efficiency of morethan fifty percent.
 32. The method of claim 31 wherein said radiator isa mantle composed of ytterbium oxide strands.
 33. The method of claim 31and further including the steps of supporting said radiator adjacent theoutlet port of a fuel supply conduit, flowing fuel through said conduitto said radiator, and igniting said flowing fuel to cause said radiatorto emit radiation in said spectral irradiance profile.
 34. The method ofclaim 31 and further including the step of providing a reflector systemfor directing the radiated flux from said radiator to said photocell.35. The method of claim 34 and further including the step of interposingradiation transmitting thermal isolation means between said radiator andsaid photocell.